Simple dual battery question

[nfpgasmask]

For those of you with solid 12V knowledge about running a dual battery setup, how important is it to keep the batteries up under the hood? My vehicle has VERY limited space under the bonnet but I do have a nice space under the back seat where I could easily build and mount a battery box. So I would have one battery under the hood and one under the back seat. I would imagine this would take 6-9 feet of cable to get up to the front of the vehicle. Does this length of cable between batteries pose any issues? It is my understanding that as long as I use proper cable, it should work ok. Any advice here?

Thanks.

 

[DiploStrat]

For those of you with solid 12V knowledge about running a dual battery setup, how important is it to keep the batteries up under the hood? My vehicle has VERY limited space under the bonnet but I do have a nice space under the back seat where I could easily build and mount a battery box. So I would have one battery under the hood and one under the back seat. I would imagine this would take 6-9 feet of cable to get up to the front of the vehicle. Does this length of cable between batteries pose any issues? It is my understanding that as long as I use proper cable, it should work ok. Any advice here?

Thanks.

My batteries are about 20 feet (40 feet round trip) away. Get yourself a decent calculator, like this one found by dwh and do your homework. The big numbers you need to know are the maximum charge rate that you can achieve (probably the output of your alternator) and the maximum discharge that you are going to face and the distance involved. Of course, the charge and discharge make be routed through different cables.

http://www.calculator.net/voltage-drop-calculator.html

N.B. Batteries inside the passenger compartment raise all manner of ventilation and fire hazard issues. Be careful!

 

[1stDeuce]

Actually, given that it is under the seat and inside the vehicle, an AGM would work well there. (You don't want acid spilling in the event of a crash, and you won't have to vent the hydrogen gas outside that way.)

If you are expecting the battery to help out with camping loads, then running smaller (#4 or #2ga) cable would be fine, and the AGM would be happy with a slight loss in voltage when being charged by the alternator. If you want it to help out more for winching, then big cable, like #1/0 cable is really ideal, and the AGM will suffer a little with higher charge voltage, but likely not enough to worry about, and still better than if it was under the hood, where it would also be hot. (Hot batteries need less charge voltage to be happy.)

So go for it! Just be safe when running the cables, and you might want to use a BIG 500A breaker at both ends of the + cable just in case it ever gets shorted.

 

[nfpgasmask]

Thanks guys. First off, maybe I wasn't clear. The second battery will be in a box under the back seat, but still on the outside of the vehicle. What I meant to say, is that under the vehicle there is a big empty space under the the location of the rear seat, that would be ideal for a battery box. So I would not have to worry about fumes or acid. A nice Die Hard/Odyssey battery pair is what I would likely go with. I am horrible at math so thanks for the calculator. I have not done all the needed homework yet on whether or not my alternator can even handle a dual battery setup. Right now I am just trying to see if there is a way I could fit two in the vehicle. The back seat location for a house battery is also ideal because I could run some 12V plugs up into the cargo area nice and clean from there, which would be cool as well.

My auxiliary power requirements would be for my Warn M8000 winch, roof rack light bar and reverse lights, power inverter and CB radio. Right now I have all that hooked up to my single battery and I have no issues, but I long to add a 12V fridge to my setup. So this is why I have been considering a dual battery install at some point.

So this is good news. I will start making a plan. If you have any good references for dual battery kits that would eliminate some of the guess work (read math) that would be excellent!

EDIT: I've always wanted one of these kits: http://www.sierraexpeditions.com/index.php?l=product_detail&p=543

Thanks again.

 

[DiploStrat]

I am a great fan of this beast: https://www.bluesea.com/products/7620/ML-ACR_Automatic_Charging_Relay_-_12V_DC_500A

-- 500w capable
-- Water protected
-- Remote override
-- Auto and bi-directional sensing (Better integration with shore and solar power.)
-- No connection to vehicle wiring.

 

[nfpgasmask]

So that unit acts as an isolator? Just add cable and batteries? Sorry for my ignorance here.

Thanks!

 

[DiploStrat]

Yes. The chain should be: Battery>fuse>cable>ACR<cable<fuse<battery.

Generically, these are called voltage sensitive relays or intelligent battery isolators (or combiners). They consist of a big relay and an intelligent controller. The differences lie in features, such as adjustability, and capacity. You want a relay rated at the output of your alternator or your winch, whichever is greater.

Review this forum, some of us (that would include me) have discussed us into the ground.

 

[Herbie]

Yes. The chain should be: Battery>fuse>cable>ACR<cable<fuse<battery.

DiploStrat has it right. Added my emphasis for the fuses. This is SOOOO important, as if you pinch/short the big cables anywhere between the two batteries, you have to make sure that BOTH will disconnect!